The invention relates generally to electro-acoustic transducers and more particularly to transducers having ceramic drivers.
As is known in the art, a transducer is a device that converts energy from one form to another. In underwater acoustic systems, transducers generally are used to provide an electrical output signal in response to an acoustic input which has propagated through a body of water, or an acoustic output into the body of water in response to an input electrical signal.
A transducer intended primarily for the generation of an acoustic output signal in response to an electrical signal is generally referred to as a projector. Conversely, a transducer designed for producing an electrical output in response to an acoustic input is called a hydrophone. Both hydrophone and projector transducers are widely employed in sonar systems used for submarine and surface ship applications.
Transducers generally include a mechanical member such as a piston, shell, or cylinder and a driver. In applications where the transducer is used as a projector, the driver is responsive to electrical energy and converts such energy into mechanical energy to drive the mechanically driven member. The driven member converts the mechanical energy into acoustic waves which propagate in the body of water. Most acoustic transducers have driver elements which use materials having either magnetostrictive or piezoelectric properties. Magnetostrictive materials change dimension in the presence of an applied magnetic field, whereas piezoelectric materials undergo mechanical deformation in the presence of an electrical field. A common piezoelectric driver is the ceramic stacked driver which is made up of individual ceramic elements which are stacked with alternating polarities. In this stacking arrangement, the ceramic stack is longitudinally polarized. Electrical drive is applied to the elements of the ceramic stack and in response, each element expands and contracts in the longitudinal direction. The individual element displacements accumulate to provide a net displacement of the stack.
A common configuration for acoustic transducers used in underwater environments is the longitudinally polarized cylindrical projector, known commonly as the Tonpilz projector. The Tonpilz projector makes use of a stack of cylindrical ceramic elements mounted between a weighted baseplate, called the tail mass, and a lighterweight movable solid metal piece with a flat circular, or piston-like, face called the head mass. A bias rod through the center of the ceramic stack connects the tail mass to the head mass. In one common configuration, the bias rod has a threaded portion at one end which mates with a complementary threaded hole of the head mass. The driver elements and tail mass are placed over the rod and secured together with a locking nut. A predetermined torque is applied to the nut for compressing, or prestressing, the ceramic elements so that they are protected from tensile forces which are generally detrimental to ceramic piezoelectrics. In some applications, the needed prestress may require a level of torque that may be difficult to administer and control.
Another projector which is commonly used when light weight, small size and/or high efficiency is needed, is the so-called flextensional transducer. One known flextensional transducer includes a rectangular ceramic driver mounted within and along the major axis of an elliptically shaped shell. Prestress is applied to the driver by compressing the shell along its minor axis, thereby extending the major axis dimension allowing a slightly oversized ceramic stack driver to be placed along the major axis. Releasing the compressive force applied to the elliptical shell places the driver in compression. With this configuration, the elliptical shell acts as a mechanical impedance transformer between the driving element and the medium, such as a body of water, in which the transducer is disposed. The dynamic excitation of the ceramic stack driver causes the stack to expand and contract. A small velocity imparted at the ends of the ceramic stack is converted to a much larger velocity at the major faces of the elliptical shell resulting in the generation of an acoustic pressure field within a medium in which the transducer is disposed. It is generally desired for good electro-acoustic efficiency that contact is made to the drive points of the shell only by the ceramic stack assembly.
One problem with applying compressive prestress to the ceramic stack drivers in a flextensional transducer relates to the earlier mentioned technique for inserting the ceramic stack within the shell. Compressing the minor axis in order to allow the major axis dimension to extend allows the slightly oversized ceramic stack driver to be placed along the major axis. However, the amount of compressive force is limited by the extent to which the shell can be compressed and is generally dependent on the geometry and material of the shell. The application of excessive force to the minor axis can cause the shell to yield, resulting in a ruptured shell.